Lambda Wars (formerly Half-Life 2: Wars) brings the struggle between the human resistance and their Combine overlords to a multiplayer real-time strategy setting. The Lambda Wars beta only requires Alien Swarm installed to play. It features 3 multiplayer game modes: Sandbox, Annihilation and Overrun, showcasing the Rebels and Combine playable factions and a full-fledged RTS interface and HUD. There is also a singleplayer tutorial and a handful of singleplayer missions.

Agree that SMGs taking down the Strider feels out of place, maybe give rebels special units armed with shotguns(with explosive slug rounds maybe?) and AR2 while upping Strider's resistance to bullets may alleviate it.

Removing the strider seems like a minus for me, perhaps improvising a rebel tank (make a variant of combine tank), but limiting everything to infantry dosen't seem a good idea. And looking excesively for realism, would mean that the Combine, etc. shouldnt be here, at all.

Instead of a tank, what about that buggy thing, or the car that you had to drive in the main game but with a mounted gun with armour piercing rounds. Or if able, you could have addons to the vehicles like in Dawn of War and one option is a rebel with a rocket launcher or something. Just an idea. That would also open a possibility to order your vehicles to run over the enemy troops or perhaps drive into the Strider's legs. Could offer new tactics.

Nope just give the rebels insurgent tactics such as being able to hide undetected for ambushes as well as sappers that plant SLAM mines and IEDs that would even the odds. Rebels can also use old surplus human military gear such as 81mm mortars and assault rifles with grenade launchers such as the G-36 with 40mm AG-36, G3 battle rifle with HK 40mm GL, AK-47s and AK-74s with 30mm GP-25 and GP-30 GLs exc. in a limited number or course but they would be cool & useful upgrades.